1. Field of the Invention
The present invention relates to zoom lens systems and image pickup apparatuses including the same, and in particular to zoom lens systems included in image pickup apparatuses such as digital still cameras, video cameras, film cameras, and broadcast cameras.
2. Description of the Related Art
With an increase in the pixel density of solid-state image pickup devices, recent image pickup apparatuses (cameras) such as video cameras and digital still cameras provided with solid-state image pickup devices have high functionality and are of small sizes. Zoom lens systems serving as image taking optical systems to be included in such image pickup apparatuses are desired to be compact with a high zoom ratio.
Examples of a zoom lens system having a high zoom ratio include a positive-lead zoom lens system, in which a first lens unit having a positive refractive power; a second lens unit having a negative refractive power; and a subsequent lens-unit set including at least one lens unit and generally having a positive refractive power are arranged in that order from an object side to an image side.
Widely known examples of a zoom lens system having a compact body and a high zoom ratio include a retractable zoom lens system, in which intervals between lens units are made to be shorter during periods when an image taking operation is not being performed than during periods when an image taking operation is being performed.
In general, the more lens elements each of the lens units in a zoom lens system has, the larger the length of each of the lens units along the optical axis becomes. This makes it difficult to reduce the retracted length of the zoom lens system. To make such a zoom lens system storable in a compact size, the thickness of each of the lens units in a retracted state should be reduced. In this sense, it is effective to reduce the number of lens elements included in the first lens unit, which tends to have a large effective diameter.
Exemplary positive-lead zoom lens systems are disclosed in U.S. Pat. Nos. 7,206,139, 6,704,149, and 6,606,200, in each of which a first lens unit includes only a negative lens element and a positive lens element. Another exemplary positive-lead zoom lens system is disclosed in U.S. Pat. No. 7,304,805, in which lens elements included in a first lens unit are composed of an anomalous dispersion material, whereby chromatic aberration is corrected well.
In general, the total size of a zoom lens system can be reduced by reducing the number of lens elements included in each of the lens units of the zoom lens system while increasing the refractive power of each of the lens units.
However, lens elements of a zoom lens system configured in such a manner tend to become thick because of the increase of refractive powers in the respective lens surfaces. Therefore, the length of the entire zoom lens system may not be reduced sufficiently and correction of various aberrations may become difficult.
In particular, if the number of lens elements included in each of the lens units is reduced, it may become difficult to correct aberrations, such as spherical aberration and coma, related to monochrome imaging performance.
Moreover, since the type of materials that can be used for making lens elements is limited, it is difficult to correct chromatic aberration. Usually, goals of size reduction of a zoom lens system and improvement of image quality are incompatible with each other and therefore it is not easy to realize both of the goals simultaneously.
In the positive-lead zoom lens system disclosed in U.S. Pat. No. 7,206,139, the first lens unit includes lens elements composed of glass materials having high refractive indices so that both the number of lens elements and the amount of aberration occurring in the positive lens element included in the first lens unit can be reduced.
However, if materials having high refractive indices are used for the lens elements of the first lens unit, the amount of chromatic aberration may become large in a zoom range near the telephoto end as the zoom ratio is increased. Such chromatic aberration cannot be corrected well easily.
The zoom lens systems disclosed in U.S. Pat. Nos. 6,704,149 and 6,606,200 each have a high zoom ratio that is realized by satisfactory correction of chromatic aberration using a diffractive optical element provided in the first lens unit. In general, however, it is difficult to manufacture diffractive optical elements.
The zoom lens system disclosed in U.S. Pat. No. 7,304,805 realizes high performance by satisfactory correction of chromatic aberration using an optical element composed of an anomalous dispersion material. The first lens unit of the zoom lens system in U.S. Pat. No. 7,304,805 includes a negative lens element, the aforementioned optical element, a positive lens element, and another positive lens element. Since the first lens unit includes three lens elements, the length of the zoom lens system when retracted so as to be stored tends to be large.
To realize satisfactory optical performance of a positive-lead zoom lens system while increasing the zoom ratio and reducing the entire size (in particular, the retracted size), it is important to appropriately design the first lens unit, which moves during zooming.
If the configuration of the first lens unit is inappropriate, it is very difficult to realize high optical performance throughout the zoom range while maintaining compactness and a high zoom ratio.